Information recording media including CD, CD-ROM, CD-R, DVD, PD, MO and MD are already extensively used as large-capacity recording media that store audio signals and video signals. Especially, photomagnetic information recording media, information in which can be accessed by using light and magnetic fields, are attracting keen interest as high-density recording media permitting rewriting of information, and research and development attempts are actively made to achieve an even greater recording density. Research and development pursuits are also actively undertaken on photomagnetic information storage apparatus that reproduce information from and store information into such photomagnetic information recording media at high speed.
Whereas conventional photomagnetic information storage apparatus use the optical modulation system by which information is recorded on recording media by optical modulation according to information, a more recent trend developing along with the increase in recording density is to use, instead of the conventional optical modulation system, a magnetic field modulation system by which information is recorded by magnetic field modulation according to information. In a common photomagnetic information storage apparatus using the magnetic field modulation system, magnetic fields modulated by using a thin film coil fabricated by a semiconductor process are generated.
In a photomagnetic information storage apparatus using the magnetic field modulation system, the temperature of the recording film of the recording medium is brought close to the Curie point by focusing on the film a laser beam for record use, and applying in that state the magnetic fields generated by a coil onto the recording film. The magnetizing direction of the recording film is thereby oriented according to the information, which is thereby recorded on the film.
In order to execute at high speed recording and reproduction of a large quantity of data with a photomagnetic information storage apparatus using such a magnetic field modulation system, it is preferable for the device to have a front illumination type configuration in which the optical system that focuses light on the recording medium and the magnetic coil that generates the magnetic field are arranged on the same side as viewed from the recording medium. Usually an annular magnetic coil is arranged between the optical system and the recording medium in such a configuration.
Further, in order to enhance the efficiency of generating magnetic fields, a configuration in which a magnetic substance layer functioning as the core of a magnetic coil is disposed between the magnetic coil and the optical system is known to be effective. For instance, Patent Reference 1 discloses a case in which a disk-shaped magnetic substance layer having a hole at the center is provided.
(Patent Document 1)
Japanese Patent Laid-Open No. 10-320863
When the magnetic coil is driven at a high frequency (e.g. 50 MHz) where such a magnetic layer is disposed, an eddy current accompanying variations in the magnetization of the magnetic layer will be generated within the magnetic layer, and this eddy current becomes a loss and invites a rise in the temperature of the magnetic substance. This temperature rise results in an increase in the temperature of the magnetic coil and, in an extreme case known as migration in which the magnetic coil becomes very thin, possibly culminating in a wiring cut-off. The rise in magnetic coil temperature also increases the resistance of the magnetic coil, leading to damage to the magnetic coil due to heat.
As a technique to reduce such a loss due to an eddy current, it is proposed to form the magnetic layer in multiple sublayers of thin films. However, as the fabrication of such a magnetic layer of a thin film multilayered structure, such sophisticated manufacturing techniques as vacuum vapor deposition and sputtering are required, and accordingly involves an increase in production cost and a drop in productivity.